1. Field of the Invention
The present invention relates to a zoom lens barrel, and more particularly, to a mechanism for a back focal distance adjustment of the zoom lens barrel during assembly of a camera.
2. Description of the Related Art
Usually, the adjustment of a focus of a zoom lens is carried out during assembly of a camera in two steps, as follows; a first step of a zoom adjustment in which the zoom lens is adjusted so that an in-focus position is not changed by a zoom operation, and a second step of a back focal distance adjustment in which the in-focus position, which has not been changed by a zoom operation, is adjusted to coincide with a film face position. The back focal distance adjustment is refered to as a back adjustment hereinafter. The zoom adjustment, i.e., the first step, is usually carried out by moving a first lens group along the optical axis, and the back adjustment, i.e., the second step, is carried out by moving all of the lens groups along the optical axis.
Conventionally, the back adjustment is carried out by moving a cam ring, by which the positions of all of the lens groups along the optical axis are determined, but if the zooming operation is automatic, i.e., is carried out by a motor and a battery installed in a zoom lens barrel, the following problems occur. Namely, to install the motor and the battery in the zoom lens barrel, the cam ring must be provided with a notch in which the motor and the battery are positioned. Note if such a notch is not provided, the motor and the battery must be disposed outside the cam ring, and the size of the zoom lens barrel is increased. This notch, however, must be large enough to ensure that the cam ring does not interfere with the motor and the battery over the whole rotation range of the cam ring, i.e., from the tele-photo end to the wide-angle end. As a result, the mechanical strength of the cam ring is lowered, and accordingly, the cam ring is easily deformed. If the cam ring is deformed, not only do the lens positions become incorrect but also a torque change occurs and a smooth movement of the lens can not be obtained, and further in an electrically driven zooming operation, a motor speed or zooming speed fluctuates during the zooming operation.